Acidity below pH 5.5 caused asphyxiation of P. reticulatus despite adequate dissolved oxygen and low levels of carbon dioxide in sugar mill wastewater. Clogging of gills with precipitated iron when acidity fell below pH 5.5 is suspected of causing asphyxiation. No fish survived below pH 4.8 even at high DO and low CO2 levels. Survival of60% occurred between pH 4.9-5.3, while all fish survived above pH 5.5 even at low DO (0.3 ppm) and high CO2 (250 ppm) levels. Primary cause of low pH and high CO; levels was biological decomposition of suspended microscopic particles of sugar cane stalks. Major source of this material is from sludge precipitated by lime during clarification of cane juice. Separation of this sludge may increase pH in impounded wastewater to tolerable levels for fish survival.